A new tribe of sigmodontine rodents is formally defined and diagnosed. The tribe contains the extant genera Abrothrix (including Chroeomys), Chelemys, Geoxus, Pearsonomys, and Notiomys, a group of taxa distributed in the central and southern Andes and lowlands both east and west of the cordillera. The new tribe presents a unique combination of characters including nasals and premaxilla usually projected anterior to the incisors, moderately trumpeted; zygomatic plate with upper free border reduced or obsolete; and third upper molar reduced and sub-cylindrical in out-line with an internal ring-like enamel fossette. Molecular phylogenetic analyses indicate that Abrothrix is sister to a clade containing the remaining genera of the new tribe.

With approximately 84 extant genera, cricetids of the subfamily Sigmodontinae (Reig 1980) are one of the most diverse and complex groups of New World mammals. Notably, new sigmodontine taxa are still being erected on the basis of both newly discovered species (e.g., Pardiñas et al. 2005) and from revisionary museum work (e.g., Weksler et al. 2006). Distributed predominantly in South America, sigmodontines also reach Central and North America, and one extant genus is endemic to the Galapagos Islands (D'Elía 2003a).

Sigmodontine genera have been arranged into different groups, most of which have been given the formal rank of tribes (e.g., Hershkovitz 1966; Reig 1980). In the last 15 years phylogenetic analyses using either or both morphological and molecular data have been used to delimit these groups (e.g., Braun 1993; Smith & Patton 1993, 1999; Steppan 1995; D'Elía 2003b; Pacheco 2003; Weksler, 2003; D'Elía et al. 2003, 2005, 2006a, 2006b), causing several instances of major reconsiderations on their limits and contents. One of the most surprising results of DNA sequences phylogenies was a clade of taxa (Abrothrix, Chelemys, Chroeomys, Geoxus, Notiomys, and Pearsonomys), distributed primarily in the South and Central Andes and traditionally considered to be typical akodontine rodents (and in some cases even considered subgenera of Akodon [e.g., Reig 1987]), which do not form part of the akodont radiation (Smith & Patton 1999; D'Elía 2003b; see also the allozyme based studies of Spotorno et al. 1990 and Barrantes et al. 1993). Smith & Patton (1999:106-107) recommended ranking this group of genera as a tribe, suggesting that an adequate name for it would be Abrotrichini. Although these authors enumerated chromosomal, protein, and parasite features that may be used to diagnose the group, they made no formal definition or diagnosis (e.g., Musser & Carleton, 2005: 1087). Here we provide such a definition and diagnosis. We base our revised classification on the results and conclusions of Carleton (1980), Reig (1980, 1987), Smith & Patton (1993, 1999), Steppan (1995), Engel et al. (1998), D'Elía (2003b), Weksler (2003, 2006), Jansa & Weksler (2004), Percequillo et al. (2004), Steppan et al. (2004), D'Elía et al. (2003, 2005, 2006a, 2006b), Musser & Carleton (2005), and Weksler et al. (2006) and our assessment of qualitative characters. We recognize a new tribe of Sigmodontinae (family Cricetidae) that we describe as follows.

Etymology.¾Waterhouse (1837: 21, footnote) coined Abrothrix, the type genus of the new tribe, from the Greek words "abros" (soft or delicate) and "thrix" (hair), which capture a key trait of these rodents that he had noted ("fur long and soft"). The tribal name is constructed by dropping the case ending from the genitive singular form of Abrothrix (ICZN, Article 29.3), or abro-trichos, to create the stem abrotrich. Thus, the family-group name becomes abrotrich + ini = Abrotrichini, not Abrothricini.

Known distribution.¾Argentina (Pliocene-Recent), Bolivia (Recent), Chile (Pleistocene-Recent), Ecuador (Recent), Peru (Recent). Species of this tribe are known from the altiplano in west-central Peru and western mountain ranges in Argentina and Bolivia to southernmost Argentina and Chile, including several austral off-shore islands and the Patagonian steppes to the Atlantic Coast (Osgood 1943; Patterson et al. 1984; Teta et al. 2006; Fig. 2).

Phylogenetic relationships.¾Phylogenetic relationships among abrotrichine genera have been studied by Smith & Patton (1999), D'Elía (2003b), and D'Elía et al. (2006a) based on both mitochondrial and nuclear DNA sequences. Data are available for all currently recognized genera, although not for all species. These studies delineate a basal dichotomy separating Abrothrix from a clade composed by the remaining abrotrichines. The distinction of both clades is marked, with both strongly supported and corroborated by patterns of morphological variation. Within the latter clade, the three predominantly fossorial abrotrichine genera (Geoxus, Notiomys, and Pearsonomys) form a clade to which Chelemys is sister. Relationships among species of Abrothrix have not been thoroughly explored as some species have not been included in any phylogenetic analysis (we are aware of on-going studies that include A. illuteus, A. lanosus, A. markhami, and A. sanborni). Available phylogenetic studies, together with some unpublished phylogenetic analyses, indicate that current generic classification of abrotrichines may require adjustment since some genera as currently delimited are not monophyletic (D'Elía, Pardiñas, and Lessa unpublished data; see also D'Elía et al. 2006a).

Remarks.¾Based on information summarized by Reig (1987) for several abrotrichine taxa (Abrothrix spp., Chelemys macronyx, Geoxus valdivianus) and data provided by Liascovich et al. (1989) for A. illuteus, and Spotorno et al. (1990) for additional species of Abrothrix, Smith & Patton (1999) suggested that a diploid number of 2n = 52 could be a synapomorphy for the abrotrichine group. While no study has tested that hypothesis, new data indicates that the 2n = 52 is not a diagnostic character of the tribe. Pearsonomys annectens has a 2n = 56 (D'Elía et al. 2006a), unpublished results of M. Gallardo indicate that some abrotrichine taxa also depart from a 2n = 52, and a population of Abrothrix olivaceus with a 2n = 44 was recently documented by Rodríguez & Theiler (2007). The karyotype of Notiomys edwardsii and some other forms remain unknown. Further studies are needed to assess abrotrichine chromosomal evolution. What is important here is to acknowledge that a 2n = 52 is not a diagnostic feature of Abrotrichini, new tribe.

The pelage of the abrotrichinos is generally slate gray or brownish, although there are some blackish (A.sanborni, Chelemys, Geoxus) and brightly colored (A.jelskii) forms. Species of Abrothrix have elongated phalli with a much reduced or no distal baculum; the genera Chelemys, Geoxus, Notiomys and Pearsonomys have a tridigitate distal baculum; the phalli of Pearsonomys and Geoxus have a prominent dorsal hood that extends beyond the terminal crater (Hooper & Musser 1964; Spotorno 1986; D'Elía et al. 2006a). Abrothrix longipilis, A. jelskii, A. olivaceus, and Geoxus valdivianus have two pairs of preputial glands (Voss & Linzey 1981). The number of preputial glands in other genera is unknown.

Since Patton & Smith (1999) first identified the abrotrichine clade, the names Abrotrichini, Abrothrichini, and their informal derivatives (including the Spanish abrotrichinos and abrotriquinos) have repeatedly appeared in the literature (e.g., D'Elía 2003; Weksler 2003; Jansa & Weksler 2004; Geise et al. 2004; Ojeda et al. 2005; Palma et al. 2005; Jayat et al. 2006). However, none of these usages can be considered as the formal designation of the tribe since no diagnosis was provided, nor did any such appearance explicitly identify the creation of a new taxon (ICZN, 1999: Articles 13 and 16). Similarly, the usage of Abrotrichini by Smith and Patton (1999: 106) cannot be considered a formal designation of the tribe because their statement -"An appropriate name for the new tribe would be Abrotrichini " [emphasis ours]- clearly indicates only a conditional proposal, and as such, it is not available (ICZN, 1999: Articles 15.1, which corresponds to Article 15 of the edition of the Code in usage when the paper was published). Musser & Carleton (2005: 1087) also considered the earlier usages of Abrotrichini to be unavailable and recommended that a "full and proper diagnosis" was needed.

The informal term "Andean clade" has also been used to refer to the abrotrichine group. We note that even though several abrotrichines have exclusively Andean distributions (e.g., A. jelskii), several others are distributed in the lowlands west and east of the Andes (e.g., A. olivaceus), while still others are not found in the Andes at all (e.g., P. annectens).

Figure 2. Geographic distribution of Abrotrichini, new tribe.

Figura 2. Distribución geográfica de Abrotrichini, nueva tribu.

Currently (Carleton & Musser 2005 with the present modification), nine tribes are recognized within Sigmodontinae; these are: Abrotrichini new tribe, Akodontini, Ichthyomyini, Oryzomyini, Phyllotini, Reithrodontini, Sigmodontini, Thomasoyini, and Wiedomyini. In addition, several genera (e.g., Abrawayaomys, Phaenomys, Wilfredomys) are considered as Sigmodontinae incertae sedis (Smith & Patton 1999; D'Elía 2003; Jorge Salazar-Bravo, personal communi-cation). We are aware of four additional genera that are to be proposed in the near future, two in the Akodontini and two in Oryzomyini. We close this contribution by offering the following revised classification of the extant Sigmodontinae.

Carleton, M.D. 1980. Phylogenetic relationships of neotomine-peromyscine rodents (Muroidea) and a reappraisal of the dichotomy within New World Cricetinae. Miscellaneous Publications Museum of Zoology, University of Michigan 157:1-146. [ Links ]

Hershkovitz, P. 1966. South American swamp and fossorial rats of the scapteromyine group (Cricetinae, Muridae), with comments on the glans penis in murid taxonomy. Zeitschrift für Säugetierkunde 31:81-149. [ Links ]

Hooper, E.T. 1959. The glans penis in five genera of cricetid rodents. Occasional Papers of the Museum of Zoology, University of Michigan 613:1-11. [ Links ]

Reig, O.A. 1980. A new fossil genus of South American cricetid rodents allied to Wiedomys, with an assessment of the Sigmodontinae. Journal of Zoology 192:257-281. [ Links ]

Reig, O.A. 1987. An assessment of the systematics and evolution of the Akodontini with the description of new fossil species of Akodon (Cricetidae: Sigmodontinae). Fieldiana: Zoology(new series) 39:347-399. [ Links ]

Voss, R.S. 1988. Systematics and ecology of ichthyomyine rodents (Muroidea): patterns of morphological evolution in a small adaptative radiation. Bulletin of the American Museum of Natural History 188:259-493. [ Links ]